1. Field of the Invention
The invention relates to audio systems, and more particularly, to audio systems using stimulus signals for measurement of transfer functions.
2. Related Art
Audio systems often include one or more applications in which transfer functions are adapted for changed conditions. Such applications typically determine the transfer function by measuring the response to a known stimulus signal, which may be a signal classified as ‘noise’ in the specific audio system. Such signals typically include white/pink noise and tone sweeps. There are multiple applications of this type of transfer function measurement.
One example application is Active Noise Cancellation in a typical audio system in which sound is to be played over one or more loudspeakers. Active noise cancellation involves adapting a cancellation filter using the transfer function of the path (also known as the secondary path) between the controlling loudspeakers and the sensing microphones. If this transfer function changes during use, the effectiveness of the noise cancellation is affected. The noise cancellation effectiveness may diminish; or worse, the system may introduce instability by adding noise instead of cancelling it. For example, the transfer function for audio in a car may be measured by the audio system manufacturer once per model of car, or by the car manufacturer once per car. During the use of the car, the transfer function may change under a variety of conditions. The transfer function may change when the occupancy changes, such as when passengers get in and out, or when cargo is added or removed. The transfer function may also change when temperature and humidity changes, or when a window is opened or closed.
Another application involving stimulus signals to measure a transfer function involves the estimation of a hearing aid feedback path. The filter that actively cancels feedback in a hearing aid operates using a model of the path from the hearing aid receiver (the little loudspeaker in the ear canal) to the external microphone. A transfer function of this model is typically measured once by the audiologist when the wearer is first given the hearing aids. Over the course of any day, a hearing aid moves around the ear canal, introducing various leaks. Over the course of weeks, wax can build up in an ear canal and change the acoustic path, especially when the receiver is plugged. Over the course of years, the ear canal can change shape and size, especially with younger wearers.
Another application involving stimulus signals to measure a transfer function involves the tuning of a concert sound system. Concert sound systems are typically tuned during sound checks prior to the concert when the venue is likely empty. As the venue fills with concertgoers with clothed bodies that absorb sound, the transfer function of the sound system changes significantly. The transfer function may change further as those people breathe air. This makes the venue warmer and more humid, which affects the speed of sound and therefore the transfer function of the sound system.
The tuning of a home theater system is another example application, which is similar to the tuning of a concert sound system. Tuning is typically done during installation of the system. The transfer function can change when the décor changes, such as the addition or removal of curtains, carpeting, and furniture, or if any of the loudspeakers need to be moved.
A similar application to both home theater tuning and active noise cancellation is the tuning of a car audio system. The transfer function between the loudspeakers and the listeners' ears can change when the occupancy, cargo, temperature, or humidity changes in the car cabin.
As noted above, applications that measure and/or adjust the transfer function in an audio system use a stimulus signal for which a response is measured. The stimulus signals typically include white or pink noise, or tone sweeps, which is unpleasant for the listener to hear. In active noise cancellation applications, the stimulus signal may cancel the purpose of the application. There is a need for a less unpleasant way of performing transfer function measurement.